Recent evidence suggesting that antidepressants may act by triggering
the birth of new neurons in the adult hippocampus,* the brain’s
memory hub, has heightened interest in such adult neurogenesis
and raised the question: Could new neurons also be sprouting up
in the parts of the adult brain involved in the thinking and mood
disturbances of depression and anxiety?

Now, scientists at the National Institute of Health’s (NIH)
National Institute of Mental Health (NIMH) have found newly born
neurons that communicate via the chemical messenger GABA (gamma-aminobutyric
acid) in adult rat cortex, seat of higher order “executive” functions,
and in the striatum, site of habits, reward and motor skill learning.
In the cortex, the new neurons appear to arise from previously
unknown precursor cells native to the area, rather than from cells
migrating in from another area. NIMH’s Drs. Heather Cameron,
Alexandre Dayer, and colleagues, report on their findings in the
January 31, 2005 Journal of Cell Biology.

Their discovery adds to the scientific debate over adult neurogenesis,
which has potential implications for understanding a variety of
brain disorders, possibly including Alzheimer’s and schizophrenia.
While most researchers agree that new neurons are generated in
the adult hippocampus and olfactory bulb, the existence of adult
neurogenesis in other brain regions remains controversial.

The NIMH team used many more markers than previous studies to
track newborn neurons as they matured and to identify the type
of neurotransmitters they secreted. The markers exploited antibody
affinities for specific proteins to tag particular cell types with
telltale color codes, visible on brain slices under fluorescence
with a laser-powered microscope.

The researchers found that the cortex and striatum were giving
birth to new, widely scattered small cells, called interneurons,
that make and secrete GABA, a neurotransmitter that dampens neuronal
activity. The new interneurons closely resembled those seen in
the hippocampus and olfactory bulb and seemed to arise at similar
rates. Interneurons are thought to play a role in regulating larger
types of neurons that make long-distance connections between brain
regions and predominate in these areas.

The NIMH team was surprised to find that the new cortex interneurons
appeared to arise from a previously unknown class of local precursor
cells rather than from cells that migrate into the area from the
subventricular zone, where other neurons  including those
seen in the striatum and olfactory bulb  originate during
adulthood. However, during development, both the cortex and striatum
precursors likely stem from common ancestor cells that somehow
retain their ability to divide and generate new GABA interneurons,
propose the researchers.

“Since antidepressants increase neurogenesis in the adult
hippocampus, they might have similar effects in the cortex, the
region probably responsible for mood dysregulation in depression,” suggested
Cameron. “But answers to such questions about regulation
and possible functions of the new neurons must await results of
future studies.”

Also participating the project were Kathryn Cleaver and Thamara
Abouantoun of the NIMH Unit on Neuroplasticity. Dr. Dayer’s
work was supported by the Swiss National Fund.

NIMH is part of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research. NIH is a component of the U.S. Department of Health and Human Services.